How does one who is able to stand but unable to bow or prostrate except while sitting pray?

Question:
May Allah reward you, Honorable Shaykh. This questioner says: If a person praying is unable to bow or prostrate except while sitting on a chair, is he required to stand for the opening takbir when he is able to do so, and then sit down afterward?

Answer:
Yes, if he is able to stand, he is required to say the opening takbir while standing. If standing becomes difficult for him, he sits on the chair or on the ground, and he is required to stand to the extent that he is able. This is in the obligatory prayer. As for the voluntary prayer, there is no blame on him; it is permissible for him to pray the voluntary prayer while sitting, even if he is able to stand. As for the obligatory prayer, no; standing is a pillar in it, provided one has the ability. So he says the takbir and stands to the extent that he is able, and if he becomes tired, he sits. Yes.